Personally, breaking alliances robs the game of fun for me. The problem is that I make strategic decisions resting upon the assumption that an alliance will not be broken.
However, if they are broken, it renders the point of the game (strategy) moot.
With the point of the game gone, then it's not entertaining, challenging, or fun anymore.
Aside from that, backstabbing is a very bad idea. Why? Vengeance. The other player will try and drag you down to hell with them if you backstab them. If you just give fair warning, they usually do not attempt to self-destruct and destroy you as well.
Furthermore, if the backstab is unsuccessful, then you are in a REALLY bad position, because the erstwhile ally is going to basically tell all your allies that you broke an alliance and soon you will be surrounded by hostile nations and unable to ally your way out.
In certain situations, I can understand the allure, but overall, the blacklisting, the danger, and the drawbacks all have me convinced it's just not worth it.